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 A183178 Numbers n such that 7*(10^(2n+1)-1)/9 - 5*10^n is prime. 2
 0, 1, 3, 7, 10, 12, 480, 949, 1945, 7548, 8923 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Original name: Numbers n such that (7*10^(2n+1)-45*10^n-7)/9 is prime. a(12) > 10^5. - Robert Price, Nov 02 2015 REFERENCES C. Caldwell and H. Dubner, "Journal of Recreational Mathematics", Volume 28, No. 1, 1996-97, pp. 1-9. LINKS Patrick De Geest, World!Of Numbers, Palindromic Wing Primes (PWP's) Makoto Kamada, Prime numbers of the form 77...77277...77 FORMULA a(n) = (A077777(n-1)-1)/2 for n>1. MATHEMATICA Do[If[PrimeQ[(7*10^(2n + 1) - 45*10^n - 7)/9], Print[n]], {n, 3000}] PROG (PARI) is(n)=ispseudoprime((7*10^(2*n+1)-45*10^n-7)/9) \\ Charles R Greathouse IV, Jun 13 2017 CROSSREFS Cf. A004023, A077775-A077798, A107123-A107127, A107648, A107649, A115073, A183174-A183187. Sequence in context: A310182 A020679 A103541 * A340604 A310183 A145004 Adjacent sequences:  A183175 A183176 A183177 * A183179 A183180 A183181 KEYWORD nonn,base AUTHOR Ray Chandler, Dec 28 2010 EXTENSIONS Name edited and a(1) = 0 inserted by M. F. Hasler, Feb 07 2020 STATUS approved

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Last modified September 24 22:33 EDT 2021. Contains 347651 sequences. (Running on oeis4.)